In selective catalytic reduction (SCR) of NOx by hydrocarbons (HCs) (referred to herein as HC-SCR), HCs are reacted with NOx, rather than with oxygen, to form nitrogen, CO2 and water according to reaction (1):{HC}+NOx→N2+CO2+H2O  (1)
Some nitrous oxide (N2O) may also be formed in reaction (1).
The competitive, undesirable non-selective reaction with oxygen is given in reaction (2):{HC}+O2→CO2+H2O  (2)
HC-SCR catalysts are also referred to as “lean NOx catalysts” (LNC) or “DeNOx catalysts”.
Whilst conversion can be improved if the exhaust gas has a relatively low oxygen content, many combustion exhaust gases produced by lean-burn internal combustion engines have relatively high oxygen content, typically 5-10% in the case of diesel exhaust gas.
“C1 HC” as used herein is not a reference to methane or some other hydrocarbon species having only one carbon atom. Rather, it is an expression adopted by researchers in the field to equate concentrations of hydrocarbon reductants so that conditions can be compared no matter whether, for example, iso-butane (C4H8) or propene (C3H6) reductant is used. For example, iso-butane is C4 and so its C1 concentration will be four times that of iso-butane per se, whereas propene is C3 and so its C1 concentration will be three times that of propene per se.
A commonly quoted HC-SCR catalyst is copper-exchanged elite, such as Cu/ZSM-5 (see Iwamoto, M., et al., 1991. “Removal of Nitrogen Monoxide through a Novel Catalytic Process”, Journal of Physical Chemistry, 95, pg. 3727-3730), which is active at relatively high temperatures, between about 350 and 500° C. However, the experimental conditions in the laboratory did not include H2O or SO2 which are ubiquitous components of combustion exhaust gas and subsequently they were found to inhibit NOx reduction.
Fe/ZSM-5 catalysts have also been studied in the laboratory, but the preparation route for the most active reported catalysts is complicated. Chen and Sachtler (Catalysis Today, 42, (1998) 73-83) obtained a Fe/ZSM-5 catalyst with Fe/Al˜1 by chemical vapour deposition (sublimation) of FeCl3 onto H-ZSM-5 and compared its activity with Fe/ZSM-5 prepared by ion exchange in an aqueous slurry using FeSO4 precursor. In the range 250-500° C. considered, the Fe/ZSM-5 catalyst prepared by sublimation was found to have a 75% NO conversion at about 350° C. when iso-butane was the reductant, whereas the best conventionally prepared catalysts showed a peak conversion of about 55% at 350° C. Activity of their more active catalyst was found to be unimpaired at high temperature in the presence of 10% H2O (Catalysis Letters 50 (1998) 125-130).
Chen and Sachtler also reported difficulty in repeating the procedure of Feng and Hall (described by the latter pair in J. Catal. 166 (1997) 368) for the preparation of “overexchanged”Fe/ZSM-5 catalysts from FeC2O4.2H2O, which was said to minimise oxidation of Fe2+ to Fe3+ and precipitation of FeOOH and Fe(OH)3 while maintaining the pH in the range 5.5<pH<7.0. Feng and Hall had reported achieving 100% conversion between 450 and 550° C. using iso-butane as reductant with their overexchanged Fe/ZSM-5. However, more recently, Feng and Hall reported that neither the initial “overexchanged” catalysts nor their results could be reproduced (Catal. Lett. 52 (1998) 13-19) (about 60% NO conversion with iso-butane at a Tmax of 425° C.) the originally observed activity being explained by some unknown property of the zeolite sample used to prepare the catalysts.
More recent investigations into Fe-ZSM-5 HC-SCR catalysts have looked at catalysts prepared by solid state ion exchange and chemical vapour deposition and concluded that an extensive washing step prior to calcination plays a key role in catalyst activity (see Heinrich et al., Journal of Catalysis, 212, 157-172 (2002)). Analysis of catalyst activity was carried out at between 523 and 823 Kelvin (250-550° C.) using iso-butane as reductant.
Fe/Y elite (FeY) was also studied as a potential redox catalyst using inter alia CO/NO as a model to compare with the activity of Cu/Y elite at 300° C. (see J. O. Petunchi et al. Journal of Catalysis, 80,403-418 (1983)).
Some transition metals/zeolites are also known for catalysing the reduction of NOx to N2 with NH3 reductant. See for example U.S. Pat. No. 4,961,917 (iron/beta elite).
Due to the selective character of reaction (1) (in practice, the maximum selectivity of HC-SCR catalysts in an exhaust system of a diesel engine is limited to about 20%, i.e., 20% of HC reacts with NOx and 80% with oxygen), HC-SCR catalysts show increasing NOx conversion rates with increasing hydrocarbon concentrations (C1 HC to NOx mole ratios between 3 and 12 are usually used in laboratory evaluations, with higher ratios resulting in better NOx conversion). The limited supply of native diesel hydrocarbons (which is actually quite low in comparison to the NOx emission levels: C1 HC to NOx in native diesel exhaust is typically below 1) may constitute a barrier in achieving higher NOx conversions, especially if the catalyst selectivity is low. Actively enriching the exhaust gases with additional HC material has been perceived as a solution to this problem. In general, such enrichment could be realized by two methods:                (i) injection of hydrocarbons, preferably diesel fuel, into the exhaust system upstream of the catalyst; or        (ii) late in-cylinder injection using common rail fuel injection, or merely late injection timing in conventional fuel injection systems.        
Exhaust systems with exhaust gas HC enrichment have been termed “active” systems. To distinguish from such active systems, exhaust systems in which HC-SCR is achieved exclusively using unburned hydrocarbons in native exhaust gas are often referred to as “passive” HC-SCR systems. Active HC enrichment does involve a certain fuel economy penalty, depending on the quantity of injected fuel. An additional oxidation catalyst may also be necessary in the active configurations to oxidize hydrocarbons which may pass through (or “slip”) the HC-SCR catalyst.
Known methods of reducing NOx in lean exhaust gas from internal combustion engines include HC-SCR catalysts, such as platinum on an alumina support, Cu/elite and silver on an alumina support; NH3 SCR (see e.g. U.S. Pat. No. 4,961,917); or NOx storage catalysts (NOx traps)—see for example EP 560991. However, a problem with each of these techniques is that none is known to be active for reducing NOx at temperatures below 150° C.
Emissions standards such as the FTP for light-duty diesel vehicles set a limit on the level of exhaust gas components CO, HC, NOx and particulate matter (PM) it is permissible to emit over the course of a standard test cycle. Such emission test cycles include emissions released immediately after the engine is switched on (often referred to as “cold start”) and since known catalytic methods of removing NOx are not active until the catalyst temperature rises sufficiently, the period between cold start and the onset of catalytic activity promoting NOx reduction can contribute greatly to the overall NOx emissions over the whole cycle.
WO 2004/094045 discloses a method of decomposing nitrogen dioxide to nitrogen monoxide in an exhaust gas of a lean burn internal combustion engine, such as a diesel engine, comprising adjusting the C1 hydrocarbon:nitrogen oxides (C1 HC:NOx) ratio of the exhaust gas to from 0.1 to 2 at above 250° C. and contacting this exhaust gas mixture with a particulate acidic refractory oxide selected from the group consisting of zeolites, tungsten-doped titania, silica-titania, zirconia-titania, gamma-alumina, amorphous silica-alumina and mixtures of any two or more thereof and passing the effluent gas to atmosphere. The particulate oxide may support a metal or a compound thereof including copper or iron.
WO 2005/088091 discloses a method of reducing NOx in exhaust gas flows of a motor vehicle, by means of a catalyst, characterised in that a NOx absorbing material is provided in the catalyst. In one embodiment, the NOx absorbing material is a elite, optionally containing oxidative metallic ions including copper and/or iron. It is suggested that a elite containing metallic ions can also oxidise NO to NO2 in addition to absorbing NOx. The method includes absorbing NO at temperatures≦20° C. and desorbing NO at increasing (unspecified) temperatures. Regions for oxidising NO can be combined with regions for reducing NOx. Such reducing regions comprise clay minerals such as bentonite, sepiolite, hectorite and montmorillonite, preferably containing basic cations such as Ba, Na, Sr, Ca and Mg to bind hydrocarbons and convert them to more reactive species, such as aldehydes. NOx reductants such as HC, CO/H2 or ammonia are disclosed. However, so far as it can be understood, the disclosure does not mention any temperatures at which the reductants are brought into contact with the catalyst. Furthermore, there are no Examples to enable the skilled person to ascertain how some of the more impressive alleged advantages are obtained, e.g. a minimum 52% NOx reduction relative to prior art engines.
JP 04-284825 discloses a method of reducing NO in an exhaust gas to N2 at above 300° C. by introducing hydrocarbons having a carbon number from 2-7 into the exhaust gas and contacting the resulting exhaust gas mixture with a metal-containing elite (metals including copper, manganese, cobalt, iron, nickel, chromium and vanadium).